Having undertaken its initial heavy-lift projects in the North Sea last year, Allseas’ Pioneering Spirit recently began to demonstrate the other aspect of its incredible capability when it started pipelaying the deepwater sections of the TurkStream gas pipeline
Much has been made of the importance to the decommissioning market of the cost-effectiveness of single-lift vessels, with Allseas’ Pioneering Spirit being the prime example.
Analyses of that market suggest that new-generation single-lift vessels (SLVs) will play a major role in reducing costs, in many cases, making decommissioning projects feasible. As one recent study by Douglas-Westwood noted of the role of SLVs, “a huge amount will depend on the success of early removal projects”.
However, large-scale decommissioning projects don’t yet occur with sufficient regularity to keep a vessel such as Pioneering Spirit permanently employed. Hence, the vessel has two roles, also operating as a pipelayer with an S-lay tension capacity of up to 2,000 tonnes, installing the heaviest and largest pipelines in the industry and doing so at a very high lay rate compared with other vessels.
The first heavy-lift project that this unique vessel undertook – lifting the Yme topsides on behalf of Repsol in the North Sea – was completed in August 2016, but in June 2017, it started working on quite a different sort of project, pipelaying on the deepwater sections of the TurkStream gas pipeline.
Other heavy-lift vessels able to undertake single lifts of massive offshore structures have been proposed, but for the time being at least, Allseas’ new vessel is a ship unlike any other, able to single lift topsides of up to 48,000 tonnes and jackets up to 25,000 tonnes. The vessel, which has a wide slot at the bow that fits around platform substructures, is equipped with a hydraulically operated topsides lifting system. For installing or removing platform jackets, the vessel has a system of two tilting lift beams at the stern.
Prior to the development of the SLV concept, installation of large offshore platforms has generally been carried out by crane vessels, and large topsides have to be installed module by module in order to ensure that the lifts remain within the capacity of the cranes. This can lead to considerable offshore hook-up costs to complete the topsides. However, if a topside can be installed with a single lift, hook-up costs can be significantly reduced. Moreover, if a topside structure is completed on land and then installed in one piece – with all of the structural, electrical, hydraulic and piping work done on land – the whole process can be executed much more quickly and efficiently.
Equally, removing large topsides using conventional crane vessels requires the topsides to be removed module by module or by time-consuming, hazardous and expensive dismantling offshore, piece by piece. When removing topsides in this way, every module must be cut loose from the surrounding modules, lift points must be reinstalled and lift rigging must be attached. Platform walkways, escape routes and safety plans must be adjusted several times during the removal process. Several ‘make safe campaigns’ are required, offshore and onshore, and at an early stage, accommodation facilities will have to be removed because they are located on the outside of the structure and will no longer be available to the crew sent to dismantle the unit.
Another important feature of the single-lift vessel concept is that, whereas removal of jackets by conventional techniques requires them to be cut up into small sections, Pioneering Spirit can remove jackets in their entirety and subsea work to the foundation piles is also minimised. Moreover, working in this way increases the economic value of the topsides and jackets once removed, because they remain intact, driving down conversion and refurbishment costs for reuse.
With Pioneering Spirit, therefore, Allseas has transformed installation and decommissioning projects and will significantly reduce the amount of offshore work associated with platform installation and decommissioning, shifting the work onshore where it is safer and more cost-effective. As Allseas’ founder and the man behind the single-lift vessel concept, Edward Heerema, has often affirmed, the key to the concept is that very large lifts substantially reduce offshore hook-up costs during topsides installation, just as during topsides decommissioning, single lifting can make laborious module removal unnecessary.
Installation of large, integrated topsides using the float-over technique was proposed in the 1970s and has been used on a number of occasions on a number of projects – but only in benign waters. In the Gulf of Mexico, Versatruss successfully used single-lifting with an innovative system of large frames on two barges, but only in calm water. Large, semi-submersible crane vessels such as those owned and operated by Heerema and Saipem have successfully carried out integrated lifts of up to 12,000 short tons (11,000 tonnes) with two cranes, but again, only in very favourable weather conditions.
Before commissioning the design and construction of the new unit, Allseas analysed in detail a number of large platforms in order to verify the capability that would be required of the vessel and to develop viable solutions for lifting and lowering structures, transporting them and skidding them ashore.
Gravity-based topsides, for which the leg spacing fits the slot on the vessel’s bow, can be lifted with pods fitted in the clamps of the topsides lift system. Platforms with a leg spacing of up to 51 m can be accommodated. Only a few of the widest gravity structures and jackets currently found in the North Sea do not fit Pioneering Spirit’s bow slot. The topsides of these platforms can be removed modularly using the tilting lifting beams.
Capable of sailing under its own power at a high transit speed and equipped with a dynamic positioning system and a motion compensation system developed inhouse by Allseas, Pieter Schelte is the only vessel in the world capable of working in more hostile environments such as the North Sea and will do so at unprecedented speed. The vessel also has light ice class, which will extend the period of time during which it can operate in the regions prone to ice. As Allseas also highlights, the unit’s extreme dimensions will also ensure that Pieter Schelte is an especially stable platform, with wave response behaviour that will be superior to semi-submersible crane vessels.
In keeping with the unit’s massive size, the machinery on board the vessel is also unique, being based on eight main diesel generators providing a total installed power of 95 megawatts providing power to a total of 12 azimuth thrusters. Underway, the huge vessel will have a maximum speed of 14 knots and will provide accommodation for more than 570 people in two-berth cabins.